既有建筑地下降水开挖条件下桩基承载特性及变形控制机理研究

Excavation under existing building is an effective mean to expand underground space that caters for the need of parking space. As piles are served as load-transfer structures of a high-rise building, bearing capacity of the existing piles will be eliminated due to excavation and dewatering beneath an existing building. Additional displacement of existing pile foundation will then be caused, and is likely to lead to incline, even collapse of the whole building if some effective measures on improvement of the pile bearing capacity are not conducted. Therefore, there is a need to analyze bearing behavior and mechanism of deformation control of piles under the condition of excavation and dewatering beneath existing building, and to optimize distribution of the supplementary piles to reduce differential settlement of an existing building. A variety of measures will be involved such as analysis of reported pile-test results, model test, numerical simulation and theoretical analysis. This research project aims to establish load transfer models of skin friction and end resistance considering time effect, and propose an analysis method to clarify time effect of pile capacity. Load transfer models of skin friction and end resistance of the piles under the condition of excavation and dewatering are also the prospective research focuses. Moreover, this study intends to propose an analysis method of existing pile response considering the effect of excavation and dewatering, and is expected to establish a novel method to analyze cooperative deformation of the pile groups composed of old existing piles and new supplementary piles. The results are expected to reveal behavior and mechanism of deformation control of piles during the whole process of excavation and dewatering stages, and the research is an attempt to provide theoretical support of the underground space development technique.

既有建筑增扩地下空间是解决城市停车空间紧张的有效手段。桩基础作为高层建筑的承力构件，地下降水开挖过程中其承载力会有所损失。若不采取有效处理措施必然会引起既有桩基附加变形，甚至造成既有建筑倾斜、整体倒塌破坏。如何分析降水开挖条件下既有桩基承载特性和变形控制机理，优化控制既有建筑差异沉降的补桩模式，是既有建筑增扩地下空间工程中亟待解决的关键科学问题。针对该难题，本项目拟采用已有试验数据分析、模型试验、数值模拟和理论分析等综合手段展开研究，建立桩侧和桩端荷载传递时效性模型，提出既有桩基承载力时效性分析方法，建立降水卸荷条件下桩侧和桩端荷载传递模型，提出考虑降水开挖影响的既有桩基承载特性分析方法，形成开挖卸荷后既有旧桩和补打新桩组合桩基协同变形控制分析方法。预期成果有望揭示既有建筑增扩地下空间全过程中既有桩基承载特性和变形控制机理，为既有建筑增扩地下空间成熟实用技术的研发和推广应用提供理论依据。

停车难问题已成为城市管理中顽疾，并逐渐成为制约城市发展的瓶颈。受城市用地资源限制，地下车库是停车的理想场所。在不影响建筑物正常使用功能前提下，对既有建筑地下空间进行增扩，既可避免既有建筑推倒重建造成的极大浪费，又可满足停车位置日益紧张的需求，具有广阔应用前景。. 本项目围绕既有建筑地下增层开挖条件下桩基变形机理及其分析方法展开研究，明确了既有桩基承载特性时效特征，建立了考虑桩-土体系变形时效性的桩基承载特性分析方法，提出了持荷条件下既有基桩间相互作用分析方法，形成了地下增层开挖全过程中考虑多因素影响的桩基承载特性量化评价方法，分析了变桩长、变桩径等不同补桩模式时群桩基础承载特性，形成了考虑桩-土体系渐进破坏的不同补桩模式下群桩基础承载特性计算方法。. 本项目研究表明，既有桩基承载力与时间近似呈双曲线关系，砂土中桩基承载力随时间增长速率小于黏土中桩基承载力随时间增长速率；开挖条件下桩侧和桩端荷载传递关系可采用考虑回弹效应的改进双曲线模型描述；开挖条件下持荷群桩间相互作用系数随沉降增加而增大，且随开挖宽度和深度增加而减小；地下增层开挖后应按照变形控制的思想设计补桩参数，开挖后变桩长、变桩径等组合桩基可有效降低群桩基础差异沉降。. 本项目相关研究解决了既有桩基承载力时间效应、开挖过程中桩基承载力损失评价和变形分析、不同补桩模式时组合桩基变形机理等关键科学问题。依托本项目，项目负责人出版专著1部，主编教材1部，参编行业标准1部，发表SCI/EI论文19篇（第一或通讯作者17篇）、核心期刊论文1篇（第一和通讯作者），以第一完成人授权发明专利10项、软件著作权4项，协助培养毕业博士研究生3名、硕士研究生2名，培养毕业硕士研究生5名，指导的2名研究生硕士学位论文获得山东省优秀硕士学位论文，超额完成了既定人才培养和科研成果目标。研究成果对提高既有建筑地下增层技术的安全性、科学性和经济性，促进相关技术推广应用具有重要理论意义和应用价值。